BACKGROUND: Umbilical artery aneurysm is a rare condition associated with increased risk for aneuploidy and fetal demise. CASE: We report a case of umbilical artery aneurysm discovered at 27 weeks of gestation in one fetus of a dichorionic, diamniotic twin pregnancy. The patient was hospitalized to monitor for expansion of the aneurysm. Corticosteroids were administered, and, after genetic amniocentesis revealed a normal karyotype, cesarean delivery was performed at 28 2/7 weeks of gestation. Pathologic examination confirmed an umbilical artery aneurysm in the cord of the affected fetus. CONCLUSION: Given the high incidence of aneuploidy associated with umbilical artery aneurysm, it is important to consider karyotype analysis of the affected fetus. If a normal karyotype is identified, early delivery may be warranted to decrease the risk of fetal demise.